Has anyone here used an aluminum CO2 tank that was designated
as Medical use for "O2" (oxygen) only? vs a CO2 tank? From what
I can tell there identical with the exception that the O2 tank
requires 2015 PSI vs just 1800 for a CO2 tank?

The valves are identical also so all equipment would work the same?

I was looking on eBay for a CO2 tank and came across a similar o2 tank
for 1/2 the price. Tipicaly most 5# co2 tanks run $50 used where a
similar O2 tank runs between $15-25.

Has anyone tried this or can they direct to a place for better info?

I plan on buying one(O2 tank) and experimenting, and will keep you
posted.

Tanks for different gasses are NOT made the same. Some require
thicker walls, some thinner, some must be made of specific allows so
as not to react with the gas they contain...you get the idea.

Each type of gas tank also has a unique "key" type valve that will
only accept a regulator designed for use with that gas. A CO2 hookup
won't attack to on oxygen tank, and vice versa. A Nitrous Oxide
hookup won't hook up to either of the other tanks. It's a failsafe
system.

An easier solution: Go to the nearest Airgas facility, or other welding
supply store & swap the tank for a tank of equal or smaller size already
filled. May be $20.00 or so compared to the cost of a replacement valve & a
fill. At Airgas we do that all the time as long as the tank is still in
test, & if you seem nice enough we'll eat the cost of the test. Good luck.

No difference in the gas itself, just the way the tanks are filled. For
industrial fills they just hook up & fill. For medical & food they vacuum
the tank down to the opposite pressure of a full pressurized tank. Example,
a full 250 cf oxygen tank is around 2500 psi, so they vacuum to -2500 psi
then fill from the vacuumed stage. Gives you around 99.95% pure ox or
whatever they're filling with. Make sense? If you have a new co2 tank the
only impurities that are gonna be in the tank is what's in the air you
breath. I wouldn't worry.